Corrosion resisting coatings



May 24; 1955 fqs m v2,799,154

CORROSION RESISTING COATINGS Filed Dec. 8, 1949 INVENTOR.

Execurmx 4 ATTORNEY United States Patent coRRosroN RESISTING COATINGSFritz J. Hansgirg, deceased, late of Yonkers, N. 1., by Josephine MariaHansgirg, executrix, Yonkers, N. Y., assignor to American Electra MetalCorporation, a corporation of Maryland Application December 8, 1949,Serial No. 131,704

2 Claims. (Cl. 204-39) This application is a continuation-in-part of theabancloned application Serial No. 18,885, filed April 5, 1948, by FritzJ. Hansgirg, deceased.

This invention relates to a method for producing by electrodeposition acontinuous, and dense corrosionresisting protective coating on ashape-retaining body of hot strength refractory metal, thereby making itpossible to use such body within hot oxidizing atmospheres in theinterior of an internal combustion engine.

Protective coatings produced by the method of the invention are usefulfor blades, vanes, nozzles, buckets and turbine wheels of gas turbines;for cylinders, tubes and nozzles of jet propulsion devices; for valvesof internal combustion engines and for other purposes where operationtemperatures up to about 700 C. and higher under corroding conditionswithin an oxidizing atmosphere are involved.

Refractory metals such as tungsten and molybdenum, and tungsten andmolybdenum alloys have high melting points, and high hot strengthproperties. However, such refractory metals form volatile oxides whenexposed to oxidizing atmospheres at elevated temperatures within therange of 700 C. and higher. For example, molybdenum oxidizes to form atrioxide which sublimes at 795 C. Tungsten oxide volatilizes at somewhathigher temperatures. Therefore, in spite of the refractory character andhot mechanical strength of shaped bodies made of tungsten andmolybdenum, they soon deteriorate when operating within a hightemperature oxidizing atmosphere.

In accordance with the invention, shape-retaining refractory bodies,such as turbine buckets of molybdenum, tungsten, alloys of molybdenumand tungsten, molybdenum base alloys and tungsten base alloys, areprovided with a corrosion resisting coating by immersing the body withina liquid electrolyte bath containing aluminum, and electrodepositingaluminum from the bath on the exterior of the body while maintaining thebath and the body at a high temperature in the range of about 900 to1100 C. and thereby causing the electrodeposited aluminum to combinewith the surface metal of the body into a substantially, denseoxygen-impervious protection coating, including at least onesubstantially continuous, oxygen-impervious coating layer consisting ofan intermetallic compound between the electrodeposited aluminum, and therefractory metal of the body.

it is, therefore, an object of the invention to provide by aneconomical, simple electrodeposition process a continuous, dense,corrosion-resisting protective coating on a shaped article ofhot-strength refractory metal, such as molybdenum, tungsten, molybdenumand tungsten alloys, molybdenum base alloys, and tungsten base alloys,thereby making it possible to operate such corrosion-resisting hotstrength shaped bodies within hot oxidizing combustion gases of aninternal combustion engine, such as a gas turbine, jet engine, or thelike.

The foregoing and further objects of the invention will be apparent fromthe following description of exemplifications thereof in connection withthe accompanying drawings wherein,

Fig. 1 is a vertical cross-sectional view showing in a diagrammaticmanner one type of equipment for producing a protectivecorrosion-resisting coating on a shaped body of refractory metal; and

Fig. 2 is a plan view of the shaped body or article shown in Fig. 1.

Referring to Fig. 1, an aluminum containing electrodeposition bath 24 isheld within a graphite crucible 10, shown open at its top. The crucibleis embedded in a suitable heat insulating material 11 held within thecontainer 12 of strong heat resistant material, such as ceramicmaterial. The electrolyte 24 is shown arranged to be heated by aninduction coil 13, such as copper tubing placed around the container 12.The coil 13 is supplied with the electric heating current of suitablehighfrequency from a source connected to the coil terminals 14, 15. Acooling fluid such as water is circulated through the current carryingcopper tubing coil 13 to prevent overheating thereof.

The article, or shaped fluid guiding body 16 to be coated is indicatedin the form of an air-foil shaped blade or bucket 16 having a root 17 bymeans of which it is secured within a suitable groove of an enginestructure such as the rotor wheel of a gas turbine. The shaped body 16is suitably held immersed within the bath 24 by an electricallyconducting structure indicated in the form of a clamping device 18provided at the lower end of an electric conductor rod 19 slidably heldin a supporting bracket 2% all of electrically conductive material. Anelectric current lead-in rod 21 of heat-resistant material such asgraphite extends through an opening in the container 12, and heatinsulation 11 into electric contact engagement with the bottom of thecarbon crucible 12, to provide a good current conducting connection tothe electrolyte bath 24. Electric insulation (not shown) may be placedaround the graphite lead-in rod 21 for insulating it from the heatinsulation 11 and container 12.

The graphite leadin rod 21 is shown provided with an external terminalclamp 28 for connecting it through a conductor to the positive terminal22 of a direct current supply source supplying the electrodepositioncurrent. The other conductor lead-in rod 19 carrying the immersed shapedbody 16 is conductively connected through conductor including switch 23to the negative terminal 29 of the direct current source.

The electrolyte bath is of such composition as to make it possible toelectrolytically deposit aluminum on the shaped body 16, whilemaintaining the electrolyte bath 24 at the proper elevated temperaturebetween about 900 to 1100 C. without excessive evaporization of themolten electrolyte.

In producing the protective coating by the procedure of the invention,the ingredients for the electrolyte bath mixture are molten within thecrucible 10 by heating to the desired temperature in the range betweenabout 900 and 1100 C. The article or shaped body 16 is first cleaned toremove surface impurities and oxide films, and the article is thenlowered and immersed into the molten electrolyte 24 within vessel 10.With the two lead-in conductors 19 and 21 connected to the negative andpositive terminal of the direct current source as shown in Fig. l, theelectrodeposition current will flow through the bath upon closing of theswitch 23, with the shaped body 16 operating as the cathode and the bath24 with the crucible 10 as the anode.

With the container or crucible 10 connected through the circuit as theanode, and the article or shaped body arca e;

16 as the cathode, the aluminum contained in the molten electrolyte willmove toward the shaped body 16 as the cathode, where they discharge as adeposit of molten aluminum. Since the temperature of the electrolyte ismaintained at a desired high level, the deposited molten aluminumcombines with the surface metal of the body into a protective coatingcompound including a continuous intermetallic compound layer between thealuminum and the refractory metal of the shaped body, which protectivecoating forms a very effective corrosion-resisting protection for thebody, and prevents access of oxygen thereto.

A highly effective procedure of the invention for providing shapedbodies of molybdenum and molybdenum base alloys with a protectivecoating that resists corrosion for prolonged periods of operation Withinhigh temperature oxidizing combustion gases, for instance, within a gasturbine, is to carry on the electrodeposition or" aluminum on thesurface of the body at a tempearture of about 1000 C. with a highcurrent density of to 20 amperes per square inch for a short time, suchas l to 20 minutes.

The procedures of the invention described herein involving placing ashape-retaining body, such as a turbine bucket, of molybdenum andmolybdenum base alloys within an aluminum containing electrolyte bathmaintained at a high temperature, such as about 1000 C. andelectrolytically depositing aluminum on the exterior of the body causesthe aluminum to combine with the underlying body metal into a protectivecoating containing a continuous dense, oxygen-impervious compoundbetween the metal of the body and the aluminum, which compound is anintermetallic compound of said metals.

The protective coating produced by the procedure of the inventioncontains the intermetallic compound layer etween the metal of the bodyand the aluminum coating metal, and a superposed exterior layer which isrich in aluminum and such coating has properties differing from those ofthe properties of the molybdenum base metal and the aluminum coatingmetal, and these protective coating layers are not soluble in mineralacids. Accordingly, surplus of free aluminum remaining on the exteriorof the protective coating formed by the process of the invention, may beremoved by simply dissolving it with dilute hydrochloric, sulphuric, ornitric acid. The protective coating layers so formed on the exterior ofthe refractory base metal body will not corrode when subjected to hightemperature oxidizing combustion gases of 800 to 900 C for long periodsof operation.

A satisfactory procedure for producing in accordance with the inventiona corrosion-resisting protective coating on the exterior of ashape-retaining body, such as a i turbine bucket, of molybdenum ormolybdenum base alloys, is as follows: The shaped molybdenum body iscleaned by dipping in a bath containing about 90% sodium hydroxide and10% sodium nitrite at a bath temperature of about 230 to 270 C. Theso-cleaned washed body is then Washed in slightly acidified hot water.The washed body is then placed within a plating bath of cryolite andsodium fluoride containing 70 mol percent AlFa (calculated), the bathbeing held within a carbon crucible or container. The deposition bathwithin the container is maintained at about 980 C. and electrolysis isperformed with a direct current of about 10 amperes per square inchsurface area of the shaped body from the bath toward the body. Thedeposition is carried on for 10 to minutes with the shaped body orbodies as the negative electrode and the crucible as the positiveelectrode. After removal from the deposition bath, the shaped body isplaced in a solution of hydrochloric acid to remove any adherent saltsand excess of free aluminum thereon, and the shaped body is thensand-blasted. To assure a perfect corrosion-resistant protective coatingon the entire exterior of the shaped body, it is again placed in theelectrolyte bath, and the electrodeposition of aluminum thereon iscarried on for another 10 to 15 minutes with the same current density,and at the same bath temperature. Following this secondelectrodeposition treatment, the shaped body is again treated with a 20%solution of hydrochloric acid to remove adhering salts and any excess offree aluminum remaining thereon, whereupon the shaped body such as aturbine bucket, is ready for assembly, in the gas turbine in which it isto be used.

X-ray diffraction analysis of the protective coating formed by theprocedure of the invention described above on a shaped molybdenum body,shows that it does not have any characteristic refraction linescorresponding to either aluminum or molybdenum, and thus establishesthat it constitutes an intermetallic compound of the two metals. Insteadof using an aluminum-fluoride electrolyte bath for producing aprotective coating by the procedure of the invention, there may be usedinstead another high temperature electrolyte which is liquid between 970and 990 C.

Another suitable electrolyte bath for practicing the invention is oneconsisting of potassium, sodium, and aluminum fluoride, which may bemaintained in a molten state at a temperature between 900 to 1000 C.,without excessive evaporization.

Another suitable electrolyte bath for practicing the invention, is oneconsisting of aluminum fluoride and barium fluoride heated to keep it ina molten state in the range between 900 and 1000 C.

Other suitable electrolyte baths for practicing the invention is aeutectic mixture of cryolite and chiolite having a melting point at 1000C.

A further suitable electrolyte for practicing the invention, is oneconsisting of about 63% aluminum fluoride and 27% sodium fluoride towhich about 10 to 12% aluminum oxide are added, such mixture meltingbetween 800 to l000 C. and forming at such temperature a clearelectrolyte from which the aluminum is precipitated through electrolysison the body 16 operating as a cathode with the aluminum oxide beingdissolved at the anode.

1f the dissolved aluminum oxide accumulates at the level of the melt incontact with the air (if no protective atmosphere is used), the aluminumoxide is moved into the melt by the stirring action caused in the meltby the high-frequency induction heating terminals. An-

H other way for removing surplus of the aluminum coating metal from theexterior of the protective coating is to pass a reverse current throughthe bath 24 with the shaped body 16 as anode, and the molten electrolytebath 24, and crucible 10 as the cathode. The current density of thereversed current can be regulated by adjusting the tap 27 on theresistor 26. In this manner surplus aluminum which has not been combinedwith the surface metal of the shaped body is redissolved in theelectrolyte bath 24. Thereupon switch 30 is opened, and the shaped body16 is removed from the electrolyte with the desired protective coatingformed thereon.

By procedures of the invention similar to those described above, shapedbodies of tungsten, molybdenumtungsten alloys and tungsten base alloyscontaining or more of the molybdenum or tungsten base, may be providedwith a corrosion resistant protective coating. Suitable base alloys ofmolybdenum and tungsten are those containing as balance titanium,zirconium, chromium and boron.

The novel principles of the invention described above will suggestvarious modifications thereof. It should, therefore, be understood thatthe invention is not limited to any of the exemplifications hereinbeforedescribed.

What is clairned is:

l. The method of providing a shape-retaining body of a base metalselected from the group consisting of molybdenum, tungsten, molybdenumbase alloys and tungsten base alloys with a protective coating whichwill remain substantially free of corrosion when exposed to hightemperature combustion gases within the interior of a combustion enginewhich method comprises placing the body in a molten electrolyte bathcontaining aluminum compounds and maintained at a high temperaturebetween about 900 C. and 1100 C., pass ing a high current densityelectric current from an anode in said bath to said body as a cathode,held heated in said bath at said high temperature, and depositingaluminum on the exterior of said body and thereby causing the depositedaluminum to combine with the base metal of the body into a protectivecoating containing at least one continuous, dense, oxygen-imperviousintermetallic compound layer between the base metal and aluminum, andremoving substantially all free aluminum remaining on the exterior ofsaid body.

2. The method of providing a shape-retaining body of a base metalselected from the group consisting of molybdenum, tungsten, molybdenumbase alloys and tungsten base alloys with a protective coating whichwill remain substantially free of corrosion when exposed to hightemperature combustion gases within the interior of a combustion enginewhich method comprises placing the body in a molten electrolyte bathcontaining aluminum compounds and maintained at a high temperaturebetween about 900 C. and 1100 C., passing a high current densityelectric current from an anode in said bath to said body as a cathode,held heated in said bath at said high temperature, and depositingaluminum on the exterior of said body and thereby causing the depositedaluminum to combine with the base metal of the body into a protectivecoating containing at least one continuous, dense, oxygen-imperviousintermetallic compound layer between the base metal and aluminum, andcontinuing the electrodeposition of aluminum on the exterior of saidbody until the intermetallic compound layer of the protective coating onthe exterior of said body has reached the thickness of at least two tofive microns, and removing substantially all free aluminum remaining onthe exterior of said body.

References Cited in the file of this patent UNITED STATES PATENTSl,488,553 Peacock Apr. 1, 1924 1,562,164 Harris Nov. 17, 1925 1,933,319Driggs et a1. 'Oct. 31, 1933 2,082,622 Fink June 1, 1937 2,109,887Mattasotti Mar. 1, 1938 2,497,119 Fink Feb. 14, 1950 2,555,372 RamageJune 5, 1951 FOREIGN PATENTS 876,759 France Aug. 17, 1942

1. THE METHOD OF PROVIDING A SHAPE-RETAINING BODY OF A BASE METALSELECTED FROM THE GROUP CONSISTING OF MOLYBDENUM, TUNGSTEN, MOLYBDENUMBASE ALLOYS AND TUNGSTEN BASE ALLOYS WITH A PROTECTIVE COATING WHICHWILL REMAIN SUBSTANTIALLY FREE OF CORROSION WHEN EXPOSED TO HIGHTEMPERATURE COMBUSTION GASES WITHIN THE INTERIOR OF A COMBUSTION ENGINEWHICH METHOD COMPRISES PLACING THE BODY IN A MOLTEN ELECTROLYTE BATHCONTAINING ALUMINUM COMPOUNDS AND MAINTAINED AT A HIGH TEMPERATUREBETWEEN ABOUT 900* C. AND 1100* C., PASSING A HIGH CURRENT DENSITYELECTRIC CURRENT FROM AN ANODE IN SAID BATH TO SAID BODY AS A CATHODE,HELD HEATED IN SAID BATH AT SAID HIGH TEMPERATURE, AND DEPOSITINGALUMINUM ON THE EXTERIOR OF SAID BODY AND THEREBY CAUSING THE DEPOSITEDALUMINUM TO COMBINE WITH THE BASE METAL OF THE BODY INTO A PROTECTIVECOATING CONTAINING AT LEAST ONE CONTINUOUS, DENSE, OXYGEN-IMPERVIOUSINTERMETALLIC COMPOUND LAYER BETWEEN THE BASE METAL AND ALUMINUM, ANDTHE REMOVING SUBSTANTIALLY ALL FREE ALUMINUM REMAINING ON THE EXTERIOROF SAID BODY.